This invention relates generally to tuners used in television (TV) receivers for converting a received TV signal into an intermediate frequency signal, and more particularly, the present invention relates to such a tuner of a double superheterodyne system used for the VHF and/or UHF band.
Recently, a double superheterodyne tuner using two local oscillators and two mixers has been developed for receiving a wide frequency range and for avoiding a tracking adjust in adjustment for every received channel. In the system, a radio frequency (RF) input signal is converted into a first intermediate frequency (IF.sub.1) signal by mixing with a output from a first local oscillator. The converted IF.sub.1 signal is filtered through a band-pass filter and converted into a second intermediate frequency (IF.sub.2) signal by mixing with an output from a second local oscillator. In this case, the higher the frequency of IF.sub.1 signal is with respect to the upper limit frequency of the receiving signal, the wider in range and the lower in disturbance the signals can be received.
However, as the signal frequency becomes IF.sub.1 higher, it becomes difficult to design, assemble and adjust the circuit. That is to say, a circuit portion including a first mixer and a second mixer is operated at very high frequency, and it is therefore necessary to assemble and adjust the circuit portion in a very precise manner. For example, when adjusting or measuring the characteristics of each of functional circuits, such as the first mixer, the second mixer, the first local oscillator, the second local oscillator, etc., it is necessary to connect to and/or separate the functional circuits from each other without any stray components.
Furthermore, in the conventional tuner, a casing containing circuit components therein is constructed of a parallelepiped box in which a plurality of plates are inserted to divide the box into several portions. In these portions, the functional circuits are located and are electrically shielded from each other by the plates. Especially in the double superheterodyne tuner, a circuit portion containing an RF amplifier, the first mixer and the first local oscillator, and a circuit portion containing the second mixer, the second local oscillator and a second intermediate amplifier are divided by a space, and each of the circuit portions is surrounded by shielding plates. In this case, a large number of shielding plates are necessary to obtain a sufficient shielding effect. Furthermore, large area of arranging the functional circuits and the space for dividing the circuit portions are required, thereby disturbing a miniaturizing the casing of the tuner.